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Tsokos Option b-2A Thermodynamics (B2.1 thru B2.5)

Essential Idea: The first law of thermodynamics relates the change in internal energy of a system to the energy transferred and the work done. The entropy of the universe tends to a maximum.

Nature Of Science: Variety of perspectives: With three alternative and equivalent statements of the second law of thermodynamics, this area of physics demonstrates the collaboration and testing involved in confirming abstract notions such as this.

International-Mindedness: The development of this topic was the subject of intense debate between scientists of many countries in the 19th century.

Understandings: The first law of thermodynamics The second law of thermodynamics Entropy Cyclic processes and pV diagrams Isovolumetric, isobaric, isothermal and adiabatic processes Carnot cycle Thermal efficiency

Applications And Skills: Describing the first law of thermodynamics as a statement of conservation of energy Explaining sign convention used when stating the first law of thermodynamics as Q = ΔU + W Solving problems involving the first law of thermodynamics

Applications And Skills: Describing the second law of thermodynamics in Clausius form, Kelvin form and as a consequence of entropy Describing examples of processes in terms of entropy change Solving problems involving entropy changes

Applications And Skills: Sketching and interpreting cyclic processes Solving problems for adiabatic processes for monatomic gases using pV5/3 = constant Solving problems involving thermal efficiency

Guidance: If cycles other than the Carnot cycle are used quantitatively, full details will be provided Only graphical analysis will be required for determination of work done on a pV diagram when pressure is not constant

Data Booklet Reference: (for monatomic gases)

Utilization: This work leads directly to the concept of the heat engines that play such a large role in modern society The possibility of the heat death of the universe is based on ever-increasing entropy Chemistry of entropy (see Chemistry sub- topic 15.2)

Aims: Aim 5: development of the second law demonstrates the collaboration involved in scientific pursuits Aim 10: the relationships and similarities between scientific disciplines are particularly apparent here

Reading Activity Questions?

Internal Energy The total kinetic energy of the molecules of the gas plus the potential energy associated with the intermolecular forces For an ideal gas, the intermolecular forces are assumed to be zero

Internal Energy Average kinetic energy of the molecules is given by where the Boltzmann constant

Internal Energy Internal energy, U, of an ideal gas with N molecules is given by and since then

Internal Energy In this equation, n is the number of moles The change in internal energy due to a change in temperature is thus

Internal Energy This formula shows that the internal energy of a fixed number of moles of an ideal gas depends only on temperature and not on the nature of the gas, its volume, or any other variable

System Means the complete set of objects under consideration Does not include the surroundings Open system – mass can enter or leave Closed system – mass cannot enter or leave Isolated system – no energy in any form can enter or leave

System State Refers to the complete set of parameters that define the system Not to be confused with states of matter (or states that matter, for that matter) Any process that change the state of an ideal gas (temperature, pressure or volume) is called a thermodynamic process Examples are adding/subtracting heat or doing work on the system

Work Done On or By A Gas Compressing a gas with a piston, the (small) work done is given by:

Work Done On or By A Gas For large changes in volume, the pressure will increase with each incremental change so it must be integrated

Work Done On or By A Gas The total work done when a gas expands by an arbitrary amount is the area under the graph of a pressure-volume diagram.

Work Done On or By A Gas The net work done is the work done by the gas minus the work done on the gas. It equals the area enclosed by the closed loop (i.e., the area between the upper and lower curves)

Thermodynamic Processes Isobaric – The gas expands or contracts under constant pressure

Thermodynamic Processes Isovolumetric (Isochoric) – The volume of the gas remains the same as pressure increases or decreases

Thermodynamic Processes Isothermal – The temperature of the gas remains the same as pressure and volume change Adiabatic – the gas does not absorb or release any thermal energy (Q = 0)

Thermodynamic Processes Isothermal – The temperature of the gas remains the same as pressure and volume change Adiabatic – the gas does not absorb or release any thermal energy (Q = 0) Key is to observe the steepness of the slope – adiabatic steep, isothermal not so steep

Thermodynamic Processes Isothermal – The temperature of the gas remains the same as pressure and volume change Adiabatic – the gas does not absorb or release any thermal energy (Q = 0)

Thermodynamic Processes Isothermal – The temperature of the gas remains the same as pressure and volume change Adiabatic – the gas does not absorb or release any thermal energy (Q = 0)

First Law of Thermodynamics When a small amount of thermal energy Q is given to a gas, the gas will absorb that energy and use it to increase its internal energy and/or do work by expanding. Conservation of energy demands that

First Law of Thermodynamics + Q = thermal energy absorbed by the gas (Qin) - Q = thermal energy lost by the gas (Qout) + W = work done by the gas (Wout) as it expands - W = work done on the gas (Win) to compress it + U = increase in internal energy/temperature - U = decrease in internal energy/temperature

What is going on here?

What is going on here? TA = 400K UA-B = 7.2 kJ UC-A = 2.2kJ Find: TB QA-B QB-C Net Work

Understandings: The first law of thermodynamics The second law of thermodynamics Entropy Cyclic processes and pV diagrams Isovolumetric, isobaric, isothermal and adiabatic processes Carnot cycle Thermal efficiency

Guidance: If cycles other than the Carnot cycle are used quantitatively, full details will be provided Only graphical analysis will be required for determination of work done on a pV diagram when pressure is not constant

Data Booklet Reference: (for monatomic gases)

Applications And Skills: Describing the first law of thermodynamics as a statement of conservation of energy Explaining sign convention used when stating the first law of thermodynamics as Q = ΔU + W Solving problems involving the first law of thermodynamics

Applications And Skills: Describing the second law of thermodynamics in Clausius form, Kelvin form and as a consequence of entropy Describing examples of processes in terms of entropy change Solving problems involving entropy changes

Applications And Skills: Sketching and interpreting cyclic processes Solving problems for adiabatic processes for monatomic gases using pV5/3 = constant Solving problems involving thermal efficiency

Essential Idea: The first law of thermodynamics relates the change in internal energy of a system to the energy transferred and the work done. The entropy of the universe tends to a maximum.

questions?

Homework #18-28

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